In accordance with an embodiment, method for generating an output signal of an amplifier having an increased signal to noise ratio includes receiving a first signal at an input terminal of the amplifier that is not part of a differential input. The input terminal of the amplifier is a sole input terminal of the amplifier. The first signal has an externally received portion and a feedback portion. A first amplified signal is generated at an output of the amplifier. High frequency components of the first amplified signal may be filtered. A feedback signal is generated at the sole input terminal of the amplifier in response to the first output signal.
1. A method for generating an output signal, comprising:providing a first amplifier having a single ended input and an output;coupling an input of a first filter to the output of the first amplifier;coupling a first input of a first comparator to the output of the first filter and coupling a second input of the first comparator for receiving a first reference voltage;coupling a first terminal of a first impedance element to an output of the first comparator and a second terminal of the first impedance element to the single ended input of the amplifier;providing a first input signal at the single ended input of the first amplifier, the first input signal including an input portion and a feedback portion;amplifying the first input signal to generate a first amplified signal at an output of the first amplifier;generating a first signal in response to the first amplified signal;comparing the first signal with the first reference voltage using the first comparator to generate a first comparison signal at the output of the first comparator; andgenerating the feedback portion of the first input signal at the single ended input of the amplifier in response to the comparison signal.
2. The method of claim 1, further including filtering the first amplified signal, using the first filter, to generate the first signal.
3. The method of claim 2, wherein filtering the first amplified signal, using the first filter, includes filtering high frequency components from the first amplified signal.
4. The method of claim 1, wherein amplifying the first input signal to generate the first amplified signal at an output of the first amplifier includes amplifying the first input signal to generate a second amplified signal and amplifying the second amplified signal to generate the first amplified signal at the output of the first amplifier.
5. The method of claim 1, further including selecting the first reference voltage to be equal to one-half a difference between an upper supply voltage and a lower supply voltage.
6. The method of claim 1, wherein providing first amplifier includes providing the first amplifier comprising:a first transistor having a control terminal, a first current carrying terminal, and a second current carrying terminal, wherein the control terminal of the first transistor serves as the single ended input of the first amplifier and the first current carrying terminal of the first transistor is coupled for receiving a first source of operating potential;a second transistor having a control terminal, a first current carrying terminal, and a second current carrying terminal, the control terminal of the second transistor coupled for receiving a bias voltage, the first carrying terminal of the second transistor coupled to the second current carrying terminal of the first transistor, and the second current carrying terminal of the second transistor coupled for receiving a second source of operating potential; andan output drive circuit having an input and an output, the input of the output drive circuit coupled to the second current carrying terminal of the first transistor and to the first current carrying terminal of the second transistor and the output of the output drive circuit serving as the output of the first amplifier.
7. A method for generating an output signal of an amplifier having an increased signal to noise ratio, comprising:providing a first gain stage having a single ended input and an output;coupling an input of a first low pass filter to the output of the first gain stage;coupling a first input of a first comparator to the output of the first low pass filter and coupling a second input of the first comparator for receiving a first reference voltage;coupling a first terminal of a first impedance element to an output of the first comparator and directly coupling a second terminal of the first impedance element to the single ended input of the first gain stage;receiving a first signal at the single ended input of the first gain stage, wherein the first signal comprises an externally received portion and a first feedback portion;generating a first amplified signal at the output of the first gain stage; andgenerating the first feedback portion at the single ended input of the first gain stage in response to the first amplified signal.
8. The method of claim 7, wherein generating the first feedback portion comprises:filtering the first amplified signal, using the first low pass filter, to generate a first filtered amplified signal;comparing the first filtered amplified signal with a first reference voltage, using the first comparator, to generate a first comparison signal; andgenerating the first feedback portion from the first comparison signal.
9. The method of claim 8, wherein filtering the first amplified signal includes filtering high frequency components from the first amplified signal.
10. The method of claim 8, wherein the first amplified signal is substantially equal to the first reference voltage.
11. The method of claim 7, wherein generating the first amplified signal at the output of the first gain stage includes:amplifying the first signal to generate an intermediate amplified signal; andamplifying the intermediate amplified signal to generate the first amplified signal.
12. The method of claim 7, further including selecting the first reference voltage to be equal to one-half a difference between an upper supply voltage and a lower supply voltage.
13. The method of claim 7, wherein the first feedback portion at the single ended input of the first gain stage sets the output voltage to be substantially equal to the first reference voltage.
14. The method of claim 7, further including generating the first signal in response to an input voltage coupled to the single ended input of the first gain stage through a first capacitor.
15. The method of claim 7, further including:providing a second gain stage having a single ended input and an output;coupling an input of a second low pass filter to the output of the second gain stage;coupling a first input of a second comparator to the output of the second low pass filter and coupling a second input of the second comparator for receiving a second reference voltage;coupling a first terminal of a second impedance element to an output of the second comparator and directly coupling a second terminal of the second impedance element to the single ended input of the second gain stage;coupling the output of the second gain stage to the first terminal of the second impedance element through a third impedance element.
16. The method of claim 15, further including:coupling the single ended input of the second gain stage to the output of the first gain stage through a fourth impedance element;coupling the output of the first gain stage to the second terminal of the first impedance element through a fifth impedance element; andcoupling a buffer to the single ended input of the first gain stage through a sixth impedance element.
17. A method for amplifying a first voltage, comprising:providing a first amplifier having a single ended input and an output;coupling an input of a first filter to the output of the first amplifier;coupling a first input of a first comparator to the output of the first filter and coupling a second input of the first comparator for receiving a first reference voltage;coupling a first terminal of a first impedance element to an output of the first comparator and coupling a second terminal of the first impedance element to the single ended input of the first amplifier;receiving the first voltage at the single ended input of the first amplifier, the first voltage comprising an input portion and a first feedback portion;generating a first amplified voltage in response to the first voltage, wherein the first amplified voltage serves as an output voltage;filtering the first amplified voltage, using the first filter, to generate a first filtered voltage;comparing the first filtered voltage with the first reference voltage to generate the first feedback portion of the first voltage.
18. The method of claim 17, wherein the first feedback portion of the first voltage sets the output voltage to be equal to the first reference voltage.
19. The method of claim 18, further including selecting the first reference voltage to increase a dynamic range of an ac voltage at the output of the first amplifier.
20. The method of claim 19, further including setting the first reference voltage to be equal to one-half a difference between an upper supply voltage and a lower supply voltage.
21. The method of claim 17, wherein filtering the first amplified voltage, using the first filter, includes filtering the first amplified voltage with a first low pass filter.
22. The method of claim 17, generating the first amplified voltage includes:amplifying the first voltage to generate a first intermediate amplified voltage; andamplifying the first intermediate amplified voltage to generate the first amplified voltage.
23. The method of claim 17, further including generating the first voltage in response to an input voltage coupled to the single ended input of the first amplifier through a first capacitor.
The present patent application is a continuation application of prior U.S. patent application Ser. No. 14/608,761, filed on Jan. 29, 2015, by Takashi Tokano, titled “PREAMPLIFER AND METHOD,” which application is hereby incorporated by reference in their entirety, and priority thereto for common subject matter is hereby claimed.
BACKGROUND
The present invention relates, in general, to electronics and, more particularly, to methods of forming semiconductor devices and structure.
In the past, audio amplifiers have been used to convert acoustic signals into electrical signals which can drive speakers. Audio amplifiers have become widely used in devices such as, for example, mobile phones, personal computers, laptops, notebooks, Tablets, etc. and typically include a microphone amplifier circuit configured to convert the audio signal into an electrical signal. One type of microphone amplifier circuit includes a digital MEMS (Micro Electro Mechanical Systems) microphone module. A digital MEMS microphone module typically includes a MEMS microphone having a terminal connected to a charge pump and another terminal connected to a preamplifier having an output connected to an analog to digital converter, which generates a digital output signal from an audio signal. A figure of merit for microphone amplifier circuits is the signal to noise ratio (SNR). A source of noise in digital MEMS microphone modules is at the preamplifier stage.
Accordingly, it would be advantageous to have microphone amplifier circuit which enhances the SNR and expands the dynamic range of the microphone amplifier circuit. It is desirable for the circuit and method to be cost and time efficient to implement.
BRIEF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying drawing figures, in which like reference characters designate like elements and in which:
FIG. 1 is a circuit schematic of a preamplifier suitable for use in a microphone in accordance with an embodiment of the present invention;
FIG. 2 is a circuit schematic of a preamplifier suitable for use in a microphone in accordance with another embodiment of the present invention;
FIG. 3 is a circuit schematic of a preamplifier suitable for use in a microphone in accordance with another embodiment of the present invention;
FIG. 4 is a circuit schematic of a preamplifier suitable for use in a microphone in accordance with another embodiment of the present invention;
FIG. 5 is a circuit schematic of a preamplifier suitable for use in a microphone in accordance with another embodiment of the present invention; and
FIG. 6 is a circuit schematic of a preamplifier suitable for use in a capacitor microphone in accordance with another embodiment of the present invention.
For simplicity and clarity of illustration, elements in the figures are not necessarily to scale, and the same reference characters in different figures denote the same elements. Additionally, descriptions and details of well-known steps and elements are omitted for simplicity of the description. As used herein current carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor or an emitter or a collector of a bipolar transistor or a cathode or an anode of a diode, and a control electrode means an element of the device that controls current flow through the device such as a gate of an MOS transistor or a base of a bipolar transistor. Although the devices are explained herein as certain n-channel or p-channel devices, or certain n-type or p-type doped regions, a person of ordinary skill in the art will appreciate that complementary devices are also possible in accordance with embodiments of the present invention. It will be appreciated by those skilled in the art that the words during, while, and when as used herein are not exact terms that mean an action takes place instantly upon an initiating action but that there may be some small but reasonable delay, such as a propagation delay, between the reaction that is initiated by the initial action and the initial action. The use of the words approximately, about, or substantially means that a value of an element has a parameter that is expected to be very close to a stated value or position. However, as is well known in the art there are always minor variances that prevent the values or positions from being exactly as stated. It is well established in the art that variances of up to about ten percent (10%) (and up to twenty percent (20%) for semiconductor doping concentrations) are regarded as reasonable variances from the ideal goal of exactly as described.
It should be noted that a logic zero voltage level (VL) is also referred to as a logic low voltage or logic low voltage level and that the voltage level of a logic zero voltage is a function of the power supply voltage and the type of logic family. For example, in a Complementary Metal Oxide Semiconductor (CMOS) logic family a logic zero voltage may be thirty percent of the power supply voltage level. In a five volt Transistor-Transistor Logic (TTL) system a logic zero voltage level may be about 0.8 volts, whereas for a five volt CMOS system, the logic zero voltage level may be about 1.5 volts. A logic one voltage level (VH) is also referred to as a logic high voltage level, a logic high voltage, or a logic one voltage and, like the logic zero voltage level, the logic high voltage level also may be a function of the power supply and the type of logic family. For example, in a CMOS system a logic one voltage may be about seventy percent of the power supply voltage level. In a five volt TTL system a logic one voltage may be about 2.4 volts, whereas for a five volt CMOS system, the logic one voltage may be about 3.5 volts.
DETAILED DESCRIPTION
Generally, the present invention provides a structure and method for increasing the signal to noise ratio of a circuit such as a preamplifier that can be used in a digital MEMS microphone. A preamplifier is typically referred to as a preamp. In accordance with an embodiment, the amplification circuit comprises a filter having an input connected to an output of a first amplifier. An output of the first filter is connected to a first input of a first comparator and a reference voltage is coupled to the second input of the first comparator. The output of the first comparator is connected to an input of the first amplifier.
In accordance with another embodiment, a method for generating an output signal is provided that comprises providing first input signal at a first input of an amplifier, wherein the first input signal includes an input portion and a feedback portion. The input signal is amplified to generate an amplified signal at the output of the amplifier and the output signal is used to generate a comparison signal by comparing a first signal with a reference signal, wherein the first signal is generated in response to the output signal. A feedback portion of the input signal is generated based on the comparison signal.
In accordance with an aspect the amplified signal is filtered to generate the first signal.
FIG. 1 is a circuit schematic of a preamplifier 10 suitable for use in a microphone such as, for example, a capacitor microphone, in accordance with an embodiment of the present invention. What is shown in FIG. 1 is an amplifier circuit 12 coupled to an output DC (direct current) control circuit 14. More particularly, amplifier circuit 12 includes a single amplifier 16 coupled to an output drive circuit 18, wherein single amplifier 16 has a single input 16I and an output 16O. Single input 16I is connected to or, alternatively, serves as an input of amplifier circuit 12 and output 16O is connected to an input 18I of output drive circuit 18. An output 18O of output drive circuit 18 serves as, or alternatively, is coupled to an output of amplifier circuit 12. Amplifier circuit 12 may be referred to as a gain stage and output drive circuit 18 may be referred to as a driver. Output 18O may be connected to an input 20I of low pass filter 20 to form a node 26, which may serve as an output of preamplifier 10. DC control circuit 14 includes a low pass filter 20 connected to a comparator 22. Low pass filter 20 has an input 20I connected to output 18O and an output 20O connected to a noninverting input of comparator 22. Comparator 22 has an inverting input coupled for receiving a reference voltage VREF and an output. An impedance element 24 has a first terminal connected to the output of comparator 22 and a second terminal connected to single input 16I. By way of example impedance element 24 is a resistor. Output 16O is connected to input 18I of drive circuit 18 and output 18O is connected to input 20I of low pass filter 20 at output 26. Output 26 may also be referred to as node 26. It should be noted that comparator 22 has a gain which is preferably designed to be low, i.e., less than about 50 decibels, so that the feedback loop created by connecting control circuit 14 to amplifier circuit 12 is stable.
In operation, an input signal VIN is coupled to or provided to input 12I and amplified by amplifier circuit 12. In response to the amplification, an amplified input signal is generated at output 18O. High frequency components of the amplified input signal are removed or filtered by low pass filter 20 to generate a filtered output signal at the noninverting input of comparator 22. A comparison signal is generated at the output of comparator 22 in response to comparing the filtered signal appearing at its noninverting input with reference voltage VREF. The comparison voltage is fed back to input 16I through impedance element 24 and serves as a feedback signal that serves to adjust the voltage at output 20O to be substantially equal to reference voltage VREF. In accordance with an embodiment, preamp circuit 10 is configured so that voltage VREF is selected to be substantially equal to one-half the difference between an upper supply voltage VDD and a lower supply voltage VSS, i.e., (VDD−VSS)/2). In accordance with embodiments in which the lower supply voltage VSS is ground, amplifier circuit 12 generates a voltage substantially equal to one-half the supply voltage VDD, i.e., VDD/2. For example, voltage VREF may be set to be 0.8 volts in response to upper supply voltage VDD being equal to 1.6 volts and the lower supply voltage VSS being equal to zero volts. In this configuration, the ac voltage at node 26 has the largest dynamic range.
FIG. 2 is a circuit schematic of a preamplifier 50 suitable for use in a microphone such as, for example a capacitor microphone, in accordance with another embodiment of the present invention. Preamplifier 50 includes an amplifier circuit 12A connected to DC control circuit 14. Amplifier circuit 12A may be referred to as a gain stage. DC control circuit 14 was described with reference to FIG. 1. Amplifier circuit 12A includes a gain stage 16A connected to output drive circuit 18 described with reference to FIG. 1. It should be noted that reference character A has been appended to reference characters 12 and 16 to distinguish embodiments of the amplifier circuits and the gain stages. Thus, amplifier circuit 12 differs from amplifier circuit 12A in the configuration of the gain stage, where the configuration of gain stage 16A may be different from that of gain stage 16. In accordance with an embodiment, gain stage 16A includes a transistor 52 connected to a transistor 54. Transistors 52 and 54 each have a control terminal, and a pair of current carrying terminals. As discussed above, the control terminals may be referred to as gate terminals, gate electrodes, or gates and the current carrying terminals may be referred to as current carrying electrodes, or one of the current carrying terminals may be referred to as a drain, a drain terminal, or a drain electrode and the other current carrying terminal may be referred to as a source, a source terminal, or a source electrode.
In the embodiment shown in FIG. 2, the drain terminal of transistor 52 is connected to the drain terminal of transistor 54 to form a node 56, the source terminal of transistor 52 is coupled for receiving a source of operating potential VSS and the source terminal of transistor 54 is coupled for receiving a source of operating potential VDD. The gate terminal of transistor 52 is coupled for receiving an input signal VIN and the gate terminal of transistor 54 is coupled for receiving a bias voltage VBIAS. It should be noted that potential VSS may be a ground potential. Node 56 is connected to input 18I of drive circuit 18 and output 18O is connected to input 20I of low pass filter 20 at node 26, which serves as an output of preamp 50.
In operation, and in response to an input signal VIN at input 16AI, i.e., the gate terminal of transistor 52, an amplified voltage is generated at node 56, which is transmitted to output 18O through drive circuit 18. High frequency components of the amplified signal at output 18O are removed or filtered by low pass filter 20 to generate a filtered output signal at the noninverting input of comparator 22. A comparison signal is generated at the output of comparator 22 in response to comparing the filtered signal appearing at its noninverting input with reference voltage VREF. The comparison voltage is fed back to input 16AI through impedance element 24 and serves as a feedback signal that serves to adjust the voltage at output 20O to be substantially equal to reference voltage VREF. In accordance with an embodiment, preamp circuit 50 is configured so that voltage VREF is selected to be substantially equal to one-half the difference between an upper supply voltage VDD and a lower supply voltage VSS, i.e., (VDD−VSS)/2). In accordance with embodiments in which the lower supply voltage VSS is ground, amplifier circuit 12A generates a voltage substantially equal to one-half the supply voltage VDD, i.e., VDD/2. For example, voltage VREF may be set to be 0.8 volts in response to upper supply voltage VDD being equal to 1.6 volts and the lower supply voltage VSS being equal to zero volts. In this configuration, the ac voltage at node 26 has its largest dynamic range.
FIG. 3 is a circuit schematic of a preamplifier 60 suitable for use in for example, a capacitor microphone, in accordance with another embodiment of the present invention. Preamplifier 60 includes an amplifier circuit 12A having an output connected to an input of a DC control circuit 14A, where an output of DC control circuit 14A is connected to an input of amplifier 12A through an impedance element 24. By way of example, impedance element 24 is a resistor. Amplifier circuit 12A including gain stage 16A connected to output drive circuit 18 have been described with reference to FIGS. 1 and 2. It should be noted that reference character A has been appended to reference character 14 to distinguish embodiments of the DC control circuits from each other. Thus, DC control circuit 14A differs from DC control circuit 14 in the configuration of the filter and the comparator, where the configuration of filter 20A may be different from the configuration of filter 20 or the configuration of comparator 14A may be different from the configuration of comparator 14. In accordance with an embodiment, low pass filter 20 includes a resistor 62 and a capacitor 54, where resistor 62 has a terminal that may be connected to or, alternatively, serves as input 20I of low pass filter 14A. Capacitor 64 has a terminal coupled for receiving a source of potential VSS and a terminal coupled to the other terminal of resistor 62 to form a node 66 that may be connected to or, alternatively, serve as an output 20AO of low pass filter 20A. Potential VSS may be, for example, ground potential.
In accordance with an embodiment, comparator 22A includes transistors 70, 72, 74, 76, and 78. Like transistors 52 and 54, transistors 70, 72, 74, 76, and 78 each have a control terminal, and a pair of current carrying terminals. As discussed above, the control terminals may be referred to as gate terminals, gate electrodes, or gates and the current carrying terminals may be referred to as current carrying electrodes, or one of the current carrying terminals may be referred to as a drain, a drain terminal, or a drain electrode and the other current carrying terminal may be referred to as a source, a source terminal, or a source electrode.
In the embodiment shown in FIG. 3, the source terminal of transistor 70 is connected to the source terminal of transistor 72 to form a node 80, the drain terminal of transistor 70 is connected to the drain terminal of transistor 74 to form a node 82, and the drain terminal of transistor 72 is connected to the drain terminal of transistor 76. Node 82 serves as the output of comparator 22A and is connected to input 16AI through impedance element 24, which impedance element 24 was described with reference to FIG. 1. The source terminal of transistor 74 is connected to the source terminal of transistor 76 to form a node 84. The source terminal of transistor 74 is coupled for receiving a source of operating potential VSS at node 84 and the source terminal of transistor 76 is coupled for receiving a source of operating potential VSS at node 84. By way of example, operating potential VSS is ground. The gate terminal of transistor 70 is coupled for receiving a reference voltage VREF and the gate terminal of transistor 72 is connected to output terminal 20AO of low pass filter 20. The gate terminal of transistor 74 is connected to the drain terminal of transistor 74 to form a diode connected transistor and the gate terminal of transistor 76 is connected to the drain terminal of transistor 76 to form a diode connected transistor.
Transistor 78 has a source terminal coupled for receiving a source of operating potential VDD, a drain terminal connected to node 80, i.e., connected to the source terminals of transistors 70 and 72, and a gate terminal coupled for receiving reference voltage VBIAS. Transistor 78 may be configured to serve as a current source.
As those skilled in the art are aware, transistors 52, 54, 70, 72, 74, 76, and 78 each have a width to length ratio. The width to length ratio of transistor 52 is given as W52/L52 and for the sake of clarity may be labeled S52. Likewise, the width to length ratio of transistor 54 is given as W54/L54 and for the sake of clarity may be labeled S54; the width to length ratio of transistor 70 is given as W70/L70 and for the sake of clarity may be labeled S70; the width to length ratio of transistor 72 is given as W72/L72 and for the sake of clarity may be labeled S72; the width to length ratio of transistor 74 is given as W74/L74 and for the sake of clarity may be labeled S74; the width to length ratio of transistor 76 is given as W76/L76 and for the sake of clarity may be labeled S76; and the width to length ratio of transistor 78 is given as W78/L78 and for the sake of clarity may be labeled S78. In accordance with an embodiment, transistors 52, 54, 74, and 78 are sized such that:
S52/S54=2*S74/S78 EQT. 1
Selecting the dimensions of transistors 52, 54, 74, and 78 in accordance with EQT 1 (Equation 1) sets the voltage at the gate of transistor 52 to be equal to the voltage at the gate of transistor 74. In accordance with an embodiment, transistors 70, 72, 74, 76, and 78 are configured such that the drain currents of transistor 70, 72, 74, and 76 are substantially equal and are about one-half the drain current of transistor 78. Thus, the gate voltages of transistors 74 and 76 are substantially equal and the gate voltages of transistors 70 and 72 are substantially equal. It should be noted that the gate voltage of transistor 70 is equal to reference voltage VREF. Setting the dimensions of transistors 52, 54, 74, and 78 in accordance with EQT 1, sets the gate voltages of transistor 52, 74, and 76 to be substantially equal. The gate voltages of transistors 54 and 78 are substantially equal to bias voltage VBIAS. Therefore the gate voltages of transistors 70 and 72 are substantially equal to reference voltage VREF. Comparator 22A is configured to have a low gain, which provides feedback loop stability and results in output voltage VOUT at node 26 being substantially equal to reference voltage VREF in response to selecting the dimensions of transistors 52, 54, 74, and 78 in accordance with EQT 1. A low gain may be, for example, less than about 50 decibels.
Thus, comparator 22A compares voltage VREF at the gate of transistor 70 with the voltage output from low pass filter 20A at the gate of transistor 72 and generates a comparison signal at node 82 which results in output voltage VOUT at node 26 being substantially equal to reference voltage VREF.
FIG. 4 is a circuit schematic of a preamplifier 90 suitable for use in, for example, a capacitor microphone in accordance with another embodiment of the present invention. Preamplifier 90 includes an amplifier circuit 12A having an output connected to an input of a DC control circuit 14B, where an output of DC control circuit 14B is connected to an input of amplifier 12A through an impedance element 24. Amplifier circuit 12A and low pass filter 20 have been described with reference to FIGS. 1 and 2. It should be noted that reference character B has been appended to reference character 14 to distinguish embodiments of the DC control circuits and to reference character 22 to distinguish the comparators from each other. DC control circuit 14B differs from DC control circuit 14 in the configuration of the comparator, where the configuration of comparator 14B may be different from the configuration of comparator 14. In addition reference character A has been appended to reference characters 82 and 84 to distinguish the connections that form nodes 82A and 84A from the connections that form nodes 82 and 84 of preamplifier 60. In accordance with an embodiment, comparator 22B differs from comparator 22A in that transistors 74 and 76 of comparator 22A have been replaced by diodes 94 and 96, respectively, and transistor 78 has been replaced by a current source 98. Diode 94 has an anode connected to the drain terminal of transistor 70 to form node 82A and diode 96 has an anode connected to the drain terminal of transistor 72. The anodes of diodes 94 and 96 may be referred to as terminals or electrodes. Likewise, the cathodes diodes 94 and 96 may be referred to as terminals or electrodes. Node 82A serves as an output of comparator 14B and is connected to input 16AI through impedance element 24. Diodes 94 and 96 each have a cathode commonly connected together to form node 84A and for receiving source of operating potential VSS. By way of example, operating potential VSS is at ground potential.
FIG. 5 is a circuit schematic of a preamplifier 100 suitable for use in, for example, a capacitor microphone, in accordance with another embodiment of the present invention. Preamplifier 100 includes an amplifier circuit 12A having an output connected to an input of a DC control circuit 14, which output of DC control circuit 14 has an output connected to an input of amplifier 12A through an impedance element 24. Amplifier circuit 12A and low pass filter 20 have been described with reference to FIGS. 1 and 2. In accordance with an embodiment, preamplifier 100 includes an energy storage element 102 having a terminal connected to input 16AI and a terminal coupled for receiving input voltage VIN. Preamplifier 100 further includes an energy storage element 104 having a terminal connected to input 16AI and a terminal connected to output 26. By way of example, energy storage elements 102 and 104 are capacitors.
The dc (direct current) operation of preamplifier 100 is similar to that described with reference to FIG. 2, wherein a feedback loop sets the voltage VOUT to be substantially equal to reference voltage VREF. Capacitors 102 and 104 serve to amplify the alternating current (ac) signal appearing at input 16AI, wherein the amplified signal appears at output or node 26.
FIG. 6 is a circuit schematic of a preamplifier 150 suitable for use in, for example, a capacitor microphone, in accordance with an embodiment of the present invention. What is shown in FIG. 6 is an amplifier circuit 12 coupled to a comparator 22 and to a low pass filter 20. As discussed with reference to FIG. 1, amplifier circuit 12 includes a single amplifier 16 coupled to an output drive circuit 18, wherein single amplifier 16 has a single input 16I and an output 16O. Single input 16I is connected to or, alternatively, serves as an input of amplifier circuit 12 and output 16O is connected to an input 18I of output drive circuit 18. An output 18O of output drive circuit 18 serves as or, alternatively, is coupled to an output 154 of amplifier circuit 12. A low pass filter 20 has an input 20I connected to output 154 and an output connected to the noninverting input of comparator 22. Comparator 22 has an output connected to input 16I through a resistor 24, wherein resistor 24 has a terminal connected to the output of comparator 22 and a terminal connected to input 16I. A capacitor 104 has a terminal commonly connected to output 154 of amplifier 12 and to input 20I of low pass filter 20. The other terminal of capacitor 104 is connected to input 16I of gain stage 16. Comparator 22 has an inverting input connected to a reference voltage VREF.
Preamplifier 150 further includes an amplifier circuit 12C comprising a single amplifier 16C coupled to an output drive circuit 18C, wherein single amplifier 16C has a single input 16CI and an output 16CO. Single input 16CI is connected to or, alternatively, serves as an input of amplifier circuit 12C and output 16CO is connected to an input 18CI of output drive circuit 18C. An output 18CO of output drive circuit 18C serves as or, alternatively, is coupled to an output 154A of amplifier circuit 12C. A low pass filter 20B has an input 20BI connected to output 154A and an output connected to the noninverting input of a comparator 22C. Comparator 22C has an output connected to input 16I through a resistor 24B, wherein resistor 24B has a terminal connected to the output of comparator 22C and a terminal connected to input 16CI of gain stage 16C. A capacitor 104B has a terminal commonly connected to the output 154A of amplifier circuit 12C and to the input 20I of low pass filter 20B. The other terminal of capacitor 104B is connected to input 16CI. Comparator 22C has an inverting input connected to a reference voltage VREF.
A buffer 106 is coupled to the input of amplifier circuit 12. More particularly, buffer 106 has a terminal coupled for receiving input voltage VIN and an output terminal connected to a terminal of energy storage element 102. The other terminal of energy storage element 102 is connected to input 16CI amplifier circuit 12C.
It should be noted that in accordance with another embodiment, gain stages 16 and 16C may be comprised of a transistor configuration such as gain stage 16A described with reference to FIG. 2, comparator 22C may be comprised of a transistor configuration such as the configuration of comparator 22B described with reference to FIG. 4, and comparator 22C may be comprised of a transistor configuration, a diode configuration, and a current source configuration such as the corresponding transistors, diodes, and current source described with reference to FIG. 4.
By now it should be appreciated that a preamplifier and a method for generating an output signal have been provided. In accordance with embodiments of the present invention, an amplifier having a single input is used to set the output dc voltage to a desired level. For example, setting the output voltage at a value substantially equal to one-half the difference of the supply voltages allows for the ac output signal to swing over a full range. In one embodiment, the dc level of the output voltage is set to a value substantially equal to VDD/2, wherein the lower supply level VSS is set at a ground potential. In accordance with another embodiment, a comparator used to generate a feedback signal is configured to have a low gain. For example, the gain may be less than 50 decibels and may be equal to or less than substantially 20 decibels.
Although specific embodiments have been disclosed herein, it is not intended that the invention be limited to the disclosed embodiments. Those skilled in the art will recognize that modifications and variations can be made without departing from the spirit of the invention. For example, junction field effect transistors, bipolar transistors or the like may be used instead of field effect transistors. In addition, the configurations of the transistors, i.e., the use of n-channel transistors and p-channel transistors, is not limited those described herein. It is intended that the invention encompass all such modifications and variations as fall within the scope of the appended claims.
